_bf_, barbels on head (for feeling); _h_, ventricle of heart; _as_, aortic bulb for regulating flow to gills; _vk_, venous sinus; _ao_, dorsal aorta; _ma_, stomach; _l_, liver; _gb_, gall cyst; _mi_, spleen; _d_, small intestine; _md_, large intestine; _a_, vent; _s_, _s_, swim bladder; _ni_, _ni_, kidney; _hl_, ureter; _hb_, bladder; _ro_, eggs (roe); _mhe_, opening of ducts from kidney and ovary. =Questions:= Are the kidneys dorsal or ventral? The swim bladder? Why? Why is the swim bladder double? Does blood enter gills above or below?] The _ovary_ lies between the intestine and the air bladder. In Fig. 220 it is shown enlarged and filled with egg masses called roe. It opens by a pore behind the vent. The silver lining of the body cavity is called the peritoneum. (See Chap. VII, Human Biology.) Is the _air bladder_ simple or partly divided in the perch? In the carp? (Fig. 220.) Is it above or below the center of the body? Why? The air bladder makes the body of the fish about as light as water that it may rise and sink with little effort. When a fish dies, the gases of decomposition distend the bladder and the abdomen, and the fish turns over. Why? Where are the _kidneys_? (Fig. 220.) Their ends unite close under the spinal column. The ureters, or tubes, leading from them, unite, and after passing a small urinary bladder, lead to a tiny urinary pore just behind the opening from the ovary. (Colored figure 4.) =The Circulation.=--The fish, unlike other vertebrates, has its breathing organs and its heart in its head. The gills have already been described. The heart of an air-breathing vertebrate is near its lungs. Why? The _heart_ of a fish is near its gills for the same reason. The heart has one auricle and one ventricle. (Colored figure 1.) [Illustration: FIG. 221.--PLAN OF CIRCULATION. _Ab_, arteries to gills; _Ba_, aortic bulb; _V_, ventricle.] Blood returning to the heart comes through several veins into a _sinus_, or antechamber, whence it passes down through a valve into the _auricle_; from the auricle it goes forward into the _ventricle_. The ventricle sends it into an _artery_, not directly, but through a _bulb_ (as, Fig. 220), which serves to maintain a steady flow, without pulse beats, into the large artery (_aorta_) leading to the gills. The arteries leading from the gills join to form a _dorsal aorta_ (_Ao_, Fig. 221), which passes backward, inclosed by the lower processes of the spinal column. After going through the _capillaries_ of the various organs, the blood returns to the heart through veins. The _color of the blood_ is given by red corpuscles. These are nucleated, oval, and larger than the blood corpuscles of other vertebrates. The blood of the fish is slightly above the temperature of the water it inhabits. [Illustration: FIG. 222.--BRAIN OF PERCH, from above.